In electrostatic printing and/or copying machines, a latent image is first produced on a latent image carrying means such as e.g. photoconductive surface of a photosensitive drum or other surface. A developer can be toner only or a mixture of toner and magnetic carrier particles. A developer is spread onto the latent image from a developer unit. Different imaging modes can be used such as Charged Area Development (CAD) or Discharged Area Development (DAD) as explained in “Electrophotography and Development Physics” 2nd edition 1988 by L. Schein (Springer Verlag) page 36. Using DAD, the toner is primarily attracted to those parts of the image which carry lower charge, typically as a result of imagewise discharge by an image exposure system, whereas the unexposed highly charged areas are not provided with toner. This way a toner image is created on the latent image carrying means. The toner is manipulated in the developer unit by means of the magnetic particles to place the toner into the correct state for printing or copying. Perfect control of the toner particles is required to prevent non-imagewise artifacts being generated in the image which are related to aspects of the developer and developer unit and not the image. A medium on which the copy or the print is to be made, e.g. sheet of paper or cardboard, is then brought in juxtaposition with the toner image and receives a transfer of toner. The toner is then heated to bond the toner to the medium on which the finished copy or print is formed. Possibly, several toner images are made on the latent image carrying means, e.g. using toners of different colours, prior to transferring and binding the latent image to the finished copy or print by heating.
In one type of printer or copier, the toner is spread onto the latent image carrying means using one or more magnetic brushes. The magnetic brush is created on a developing roller being part of the development unit which provides toner to the latent image carrying means.
In particular, in case two component development systems using a developer comprising a mixture of (reusable) magnetic carrier particles and non-magnetic pigmented toner or toner particles are used for making a permanent image, these developing rollers comprise an internal magnet roller or discrete internal magnet configuration of permanent magnets or electromagnets and an outer sleeve, being the developing sleeve, which can rotate with or independently of the internal magnet configuration.
The permanent magnets typically may comprise rubber bond magnets or sintered rare earth magnets or combinations thereof.
Transport of toner is typically achieved by rotating the outer sleeve while the internal magnetic core remains static but alternative configurations exist where the internal magnet configuration is rotated in addition to a rotation of the sleeve.
The magnetic carrier particles, dressed with toner particles that are attached by electrostatic forces, form bead chains in interaction with the magnetic field. The bead chains form a “brush”.
Most printers of this type use developing systems with a single development roller forming a simple magnetic brush (hereinafter referred to as mono-roll development systems).
Recently the need for high speed in combination with high quality has become a requirement for how new electrophotographic devices are developed. For example, the existing Xeikon presses operate in speed ranges from 90 to 240 mm/s and are based on mono-roll development systems. Since the success of high quality digital printing, the need for higher printing speeds for some market segments is increasing, but without making any compromise with respect to image quality and print flexibility. This means that the new machines should be capable of doing at least the same what the existing engines can do, but at a higher speed.
There are several patent publications dealing with this challenge, but these originate mainly from the field of black and white printing, where completely other image quality criteria are valid.
Known development systems suitable for high speed printing comprise multiple development rollers. In some of these known systems at least one of the development rollers rotates in the opposite direction to the remaining developing rollers. In the remaining of this application, we will refer to development systems with two development rollers as dual-roll development systems.
In application U.S. Pat. No. 6,879,800 a dual roll development system of a type is described. With respect to its mechanical design, this is suitable for the apparatus, method and for use with the toner of the current invention.
In DE 19,609,104 it is pointed out that this type of development unit (having at least two opposite rotating magnetic rollers), can be used for high speed printing (600-1800 mm/s).
One of the ways to reach high development speeds with enough toner density on the substrate and low amounts of background has been described in U.S. Pat. No. 7,090,956. This application has been typically developed for black and white high speed printing (1800 mm/s). In this patent the toner used in the dual roll development system is described as “available toners which are generally used”. The high speeds mentioned in that application are perfectly consistent with a binary exposing device that only creates two electrostatic potential states at the surface; one that attracts charged toner and the other that repels charged toner. In digital color printing higher screen rules and multilevel exposure increases significantly the quality of the images and therefore multi level exposure LED or laser devices are often used. This means that on a specific location on the photoconductor surface different amounts of toner can be developed depending upon the amount of light that has been sent to this specific location after charging the photoconductor drum.
The U.S. Pat. No. 7,090,956 deals with a dual roll concept that has been evaluated in the application area of “high speed and high quality full colour printing”. The unit has been designed to run off line at speeds of higher than 1000 mm/s, but for doing the real high quality printing tests, the actual available hardware platform could only reach printing speeds in the range of 90-600 mm/s. Doing these tests and using general toner formulations as described in application U.S. Pat. No. 7,090,956, we have observed a new type of image defect that was completely unknown and which has not mentioned in any previous patent application. We also did not observe anything similar when we evaluated these toner systems in the regular Xeikon printing platform which uses a developer unit with only one magnetic roller whereby the rotation goes into the same direction as the photoconductor drum. In evaluating the dual roll developing some new effect has been created with or on the photoconductor drum ending up with very uneven, non-uniform screened images. It is well known in the field of toner that additives that are not fully attached to the toner surface can generate some deposition onto the photoconductor.
In the application U.S. Pat. No. 6,878,499 is taught how to determine the amount of loose additives. It is also taught that a toner system is aimed at whereby at least 40% of the additives stay attached onto the surface under certain test conditions. When we applied this test method in a similar mode to a regular shape modified toner we found that 50% of the additives stayed onto the surface.
It is also well known in the field of electrophotographic printing that shape modified toner offers some big advantages when used in a printing process. The mobility of the toner is increased resulting in better transfer and higher image quality. There is therefore a need in the art for a shape modified toner system that brings the full advantage combination of dual roll development together with a shape modification.